PETM Weirdness

The Paleocene-Eocene Thermal Maximum (PETM) was a very weird period around 55 million years ago. However, the press coverage and discussion of a recent paper on the subject was weirder still.

For those of you not familiar with this period in Earth’s history, the PETM is a very singular event in the Cenozoic (last 65 million years). It was the largest and most abrupt perturbation to the carbon cycle over that whole period, defined by an absolutely huge negative isotope spike (> 3 permil in 13C). Although there are smaller analogs later in the Eocene, the size of the carbon flux that must have been brought into the ocean/atmosphere carbon cycle in that one event, is on a par with the entire reserve of conventional fossil fuels at present. A really big number – but exactly how big?

The story starts off innocently enough with a new paper by Richard Zeebe and colleagues in Nature Geoscience to tackle exactly this question. They use a carbon cycle model, tuned to conditions in the Paleocene, to constrain the amount of carbon that must have come into the system to cause both the sharp isotopic spike and a very clear change in the “carbonate compensation depth” (CCD) – this is the depth at which carbonates dissolve in sea water (a function of the pH, pressure, total carbon amount etc.). There is strong evidence that the the CCD rose hundreds of meters over the PETM – causing clear dissolution events in shallower ocean sediment cores. What Zeebe et al. come up with is that around 3000 Gt carbon must have been added to the system – a significant increase on the original estimates of about half that much made a decade or so ago, though less than some high end speculations.

Temperature changes at the same time as this huge carbon spike were large too. Note that this is happening on a Paleocene background climate that we don’t fully understand either – the polar amplification in very warm paleo-climates is much larger than we’ve been able to explain using standard models. Estimates range from 5 to 9 deg C warming (with some additional uncertainty due to potential problems with the proxy data) – smaller in the tropics than at higher latitudes.

Putting these two bits of evidence together is where it starts to get tricky.

First of all, how much does atmospheric CO2 rise if you add 3000 GtC to the system in a (geologically) short period of time? Zeebe et al. did this calculation and the answer is about 700 ppmv – quite a lot eh? However, that is a perturbation to the Paleocene carbon cycle – which they assume has a base CO2 level of 1000 ppm, and so you only get a 70% increase – i.e. not even a doubling of CO2. And since the forcing that goes along with an increase in CO2 is logarithmic, it is the percent change in CO2 that matters rather than the absolute increase. The radiative forcing associated with that is about 2.6 W/m2. Unfortunately, we don’t (yet) have very good estimates of background CO2 levels in Paleocene. The proxies we do have suggest significantly higher values than today, but they aren’t precise. Levels could have been less than 1000 ppm, or even significantly more.

If (and this is a key assumption that we’ll get to later) this was the only forcing associated with the PETM event, how much warmer would we expect the planet to get? One might be tempted to use the standard ‘Charney’ climate sensitivity (2-4.5ºC per doubling of CO2) that is discussed so much in the IPCC reports. That would give you a mere 1.5-3ºC warming which appears inadequate. However, this is inappropriate for at least two reasons. First, the Charney sensitivity is a quite carefully defined metric that is used to compare a certain class of atmospheric models. It assumes that there are no other changes in atmospheric composition (aerosols, methane, ozone) and no changes in vegetation, ice sheets or ocean circulation. It is not the warming we expect if we just increase CO2 and let everything else adjust.

In fact, the concept we should be looking at is the Earth System Sensitivity (a usage I am trying to get more widely adopted) as we mentioned last year in our discussion of ‘Target CO2‘. The point is that all of those factors left out of the Charney sensitivity are going to change, and we are interested in the response of the whole Earth System – not just an idealised little piece of it that happens to fit with what was included in GCMs in 1979.

Now for the Paleocene, it is unlikely that changes in ice sheets were very relevant (there weren’t any to speak of). But changes in vegetation, ozone, methane and aerosols (of various sorts) would certainly be expected. Estimates of the ESS taken from the Pliocene, or from the changes over the whole Cenozoic imply that the ESS is likely to be larger than the Charney sensitivity since vegetation, ozone and methane feedbacks are all amplifying. I’m on an upcoming paper that suggests a value about 50% bigger, while Jim Hansen has suggested a value about twice as big as Charney. That would give you an expected range of temperature increases of 2-5ºC (our estimate) or 3-6ºC (Hansen) (note that uncertainty bands are increasing here but the ranges are starting to overlap with the observations). ALl of this assumes that there are no huge non-linearities in climate sensitivity in radically different climates – something we aren’t at all sure about either.

But let’s go back to the first key assumption – that CO2 forcing is the only direct impact of the PETM event. The source of all this carbon has to satisfy two key constraints – it must be from a very depleted biogenic source and it needs to be relatively accessible. The leading candidate for this is methane hydrate – a kind of methane ice that is found in cold conditions and under pressure on continental margins – often capping large deposits of methane gas itself. Our information about such deposits in the Paleocene is sketchy to say the least, but there are plenty of ideas as to why a large outgassing of these deposits might have occurred (tectonic uplift in the proto-Indian ocean, volcanic activity in the North Atlantic, switches in deep ocean temperature due to the closure of key gateways into the Arctic etc.).

Putting aside the issue of the trigger though, we have the fascinating question of what happens to the methane that would be released in such a scenario. The standard assumption (used in the Zeebe et al paper) is that the methane would oxidise (to CO2) relatively quickly and so you don’t need to worry about the details. But work that Drew Shindell and I did a few years ago suggested that this might not quite be true. We found that atmospheric chemistry feedbacks in such a circumstance could increase the impact of methane releases by a factor of 4 or so. While this isn’t enough to sustain a high methane concentration for tens of thousands of years following an initial pulse, it might be enough to enhance the peak radiative forcing if the methane was being released continuously over a few thousand years. The increase in the case of a 3000 GtC pulse would be on the order of a couple of W/m2 – for as long as the methane was being released. That would be a significant boost to the CO2-only forcing given above – and enough (at least for relatively short parts of the PETM) to bring the temperature and forcing estimates into line.

Of course, much of this is speculative given the difficulty in working out what actually happened 55 million years ago. The press response to the Zeebe et al paper was, however, very predictable.

The problems probably started with the title of the paper “Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming” which on it’s own might have been unproblematic. However, it was paired with a press release from Rice University that was titled “Global warming: Our best guess is likely wrong”, containing the statement from Jerry Dickens that “There appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models”.

Since the know-nothings agree one hundred per cent with these two last statements, it took no time at all for the press release to get passed along by Marc Morano, posted on Drudge, and declared the final nail in the coffin for ‘alarmist’ global warming science on WUWT (Andrew Freedman at WaPo has a good discussion of this). The fact that what was really being said was that climate sensitivity is probably larger than produced in standard climate models seemed to pass almost all of these people by (though a few of their more astute commenters did pick up on it). Regardless, the message went out that ‘climate models are wrong’ with the implicit sub-text that current global warming is nothing to worry about. Almost the exact opposite point that the authors wanted to make (another press release from U. Hawaii was much better in that respect).

What might have been done differently?

First off, headlines and titles that simply confirm someone’s prior belief (even if that belief is completely at odds with the substance of the paper) are a really bad idea. Many people do not go beyond the headline – they read it, they agree with it, they move on. Also one should avoid truisms. All ‘models’ are indeed wrong – they are models, not perfect representations of the real world. The real question is whether they are useful – what do they underestimate? overestimate? and are they sufficiently complete? Thus a much better title for the press release would have been more specific “”Global warming: Our best guess is likely too small” – and much less misinterpretable!

Secondly, a lot of the confusion is related to the use of the word ‘model’ itself. When people hear ‘climate model’, they generally think of the big ocean-atmosphere models run by GISS, NCAR or Hadley Centre etc. for the 20th Century climate and for future scenarios. The model used in Zeebe et al was not one of these, instead it was a relatively sophisticated carbon cycle model that tracks the different elements of the carbon cycle, but not the changes in climate. The conclusions of the study related to the sensitivity of the climate used the standard range of sensitivities from IPCC TAR (1.5 to 4.5ºC for a doubling of CO2), which have been constrained – not by climate models – but by observed climate changes. Thus nothing in the paper related to the commonly accepted ‘climate models’ at all, yet most of the commentary made the incorrect association.

To summarise, there is still a great deal of mystery about the PETM – the trigger, where the carbon came from and what happened to it – and the latest research hasn’t tied up all the many loose ends. Whether the solution lies in something ‘fundamental’ as Dickens surmises (possibly related to our basic inability to explain the latitudinal gradients in any of the very warm climates) , or whether it’s a combination of a different forcing function combined with more inclusive ideas about climate sensitivity, is yet to be determined. However, we can all agree that it remains a tantalisingly relevant episode of Earth history.

@140 “Bob” If it’s so easy, how about your 1-2 paragraph proof of CO2 warming?

Prior to Keeling, the concentration of carbon dioxide in the atmosphere was thought to be affected by constant variability. Keeling had perfected the measurement techniques and observed strong diurnal behaviour with steady values of about 310 ppm in the afternoon at three locations (Big Sur near Monterey, the rain forests of Olympic Peninsula and high mountain forests in Arizona). By measuring the ratio of two isotopes of carbon, Keeling attributed the diurnal change to respiration from local plants and soils, with afternoon values representative of the “free atmosphere”. By 1960, Keeling and his group established the measurement record that was long enough to see not just the diurnal and seasonal variations, but also a year-on-year increase that roughly matched the amount of fossil fuels burned per year.

“No sophisticated model has been able to adequately explain changes in late Quaternary climate (or other intervals) with CO2 as an internal parameter (i.e., as a coupled feedback that responds to external forcing). Instead, CO2 (and usually methane) are prescribed as external parameters.”

Your statement gets at the heart of an important issue. The coupled models that are being used for these types of deep time geological process experiments are essentially weather prediction models with some modifications, and are being employed to try and solve problems that they were not designed to solve. Why should we expect them to faithfully reproduce many of the important non-linear emergent properties within the earth system, when many of the important geological, biological, and biogeochemical processes are absent from their code? If one wants to attempt to test certain physical hypotheses of the short-term effect of changes in atmospheric composition on weather and climate (modern climate prediction problems), that is fine, but it is unfair to expect more than the current generation of models are coded to deliver. Numerical models would essentially need to accurately depict the geological and biological evolution of the entire earth system with all its feedbacks between its various components, to even have a chance to produce many of the complex phenomena we observe in deep time climate. Can you imagine a dynamic model that numerically simulates the ES of the entire Cenozoic, where process such as crustal deformation, basin subsidence and deposition, dynamic fluid flow through the rocks, hydrocarbon generation, methane hydrate formation, benthic and pelagic ecology, and all other sorts of goodies were realistically depicted by a numerical model? Interestingly, Gavin seems to be urging usage of the “Earth System” approach, and I think may be hinting that he favors the ambitious goal to build such an integrated earth system model. (I hope he will comment in this.) In any case, I think your view of the role of numerical models is in error. They are simply tools that allow certain well-defined hypotheses to be tested numerically. They are not all-inclusive earth system prediction tools.

In response to some posts and emails … and to emphasize precise wording … the quote that initiated all this “weirdness” was *In a nutshell, there appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models.*

This should not be equated with *climate models are fundamentally wrong* — it’s the coupling of temperature and carbon that appears to be wrong when we examine intervals of Earth’s past — presumably because an important component of the climate system is either missing or incorrectly parameterized in existing models.

Is this crucial for understanding climate change over the next 100 years? Can the discrepancies be solved with a (future) sophisticated model that has carbon fluxes as internal responses to the system (plus/minus other potentially interesting feedbacks such as peat oxidation and hurricanes)? These are great questions.

Could the large and rapid (relatively speaking) rise in GHGs have been initiated by a meteor strike? The idea being that the meteor strike, occurs in a geographically sensitive area (say, continental margins, deep seas, etc) and destabilises deposits of methane hydrates. That could get the ball rolling, so to speak. Presumably there would be geological evidence of the isotopic variety, if meteor strike(s) occurred before or during the PETM.

“… the volcanic source of CO2 is depleted in 14C, but not in 13C. The source of the CO2 rise is fully depleted in 14C, but is also depleted (some, a large amount as these things go) in 13C … i.e., it is very (greater than 50 ky) old carbon from a biological source (fossil fuels, carbonate rocks).

While much work has been done in the last 10-15 years, it does not change the basic picture he documented back then (follow up his citations to the professional literature). The basics of this case were established in the late 1950s and early 1960s. Keeling’s first papers on observing CO2 included the isotopic composition for just this reason.”
— end excerpt —

Pine Island Glacier is thinning 4x faster than it was a decade ago. To some degree this might answer my question on the formation of the worlds three main ice sheets (WAIS,EAIS and Greenland) and when they formed and how stable they are. Its looking worrying for to go from 600 year melt rate to 100 is alarming some might suggest but with the warming in the pipeline to come it might disappear in even less time then it presently is.

The world has never experienced warming at this rate, its unprecedented in the history of the earth for 200 years is not even a geological time.

When you start using more of the long term, 150+ years, more original data, then we can talk. Establishing significant correlation C02 and “global temperature” is NOT a simple statistics exercise.

P.S. Having had graduate & post grad work in radiation heat transfer, as well as working in that area, I’m well aware of the Far to Near IR region of the atmosphere.

A response like this tells me you’re going to ignore anything anyone says that might threaten your fixed views on this subject. You asked me to explain AGW in two paragraphs; I did it, you reject it. And why? Because you’ve bought into the denier “the NASA data is tampered with!” line. Gimme a break. FYI, I get major contributions from CO2 with Hadley CRU and UAH TLT data as well. And global data is more relevant than data from central England, or even from Scandinavia. As a statistics minor, I know that much.

Jonas Salk once told me he solved the polio virus by imagining he was the virus, and what in the human body might hurt him. Einstein and many if not most scientists play in the same way with though experiments. The question of mechanism in PETM and Eocene optimum remains interesting and is an important key to building better models to get us closer to modeling the events more accurately.

I have had success presenting the contexts of ‘models are always wrong’ so I can relate to your expression. Unfortunately, I also know how such things get spun out of context.

I believe we are still underestimating the potentials and rapidity of change, as well as the economic consequences.

My guess is that the combined effects of multiple feedbacks in the past amplified the total effect through the resonance of one feedback upon another. Almost like a fundamental frequency in a structure amplifying upon itself, once disturbed, at the right frequency (see galloping gertie)

The amplification waves are resonating on a slower time scale than physical vibration but the effect is likely the same.

This would occur until a peak equilibrium is reached, or a system break/state change and then natural processes might mitigate/ameliorate the peak forcing over time with the main mechanism being Milankovitch cycles combining with earth processes.

Of course I don’t have a clue what these mechanisms are? Sort of like looking at and result and saying: wow, something caused that, but I can’t see the cause. But I would love to sit in on a discussion from this perspective just to see what pops up.

There is an indication of a temporal high end state (or max ceiling), based on planetary and atmospheric configuration. The questions seem to be what are the component mechanisms, and what happens around the high end thermal equilibrium? Maybe imagining this from that perspective might yield new insights from which one might work backwards towards our current state and discover new mechanisms to model?

Thanks to Ellen Thomas (#128) and Jerry Dickens (#143). In my quick scan I didn’t catch the link to ‘earth system sensitivity’ and would like to read more.

One of the important points in all these discussions is the concept of feedback. To an atmospheric scientist, changes in earth surface conditions are not considered as feedbacks, because they change what are typically considered the boundary conditions.

Slow changes to the system should really be described by a different term, like climate memory or inertia.

The fundamental point of the PETM study is that the slow changes tend to amplify the change beyond the temperature one would expect from merely a change in capture of longwave radiation.

hmmm, this idea makes me think… upon reflection, maybe this should be looked at exactly like a physical vibration since it actually is a brand of vibration on the molecular level resonating on the climate system in years, decades per cycle based on energy increases in the forcing levels contained within the sphere of earth atmosphere, a wonderful place to resonate effects inside a concave GHG shell. We need to slice up the components and look at how they resonate inside the shell???

This is one of the reasons I was intrigued by Swanson & Tsonis, 2009, though I think they didn’t have enough components involved yet to get down and get decadal (possibly a case of premature publication, or media mania ;)

Hank I’m obviously talking about the Zeebe paper that is the topic of *this post*. Also of interest is the recent finding by Swanson that was largely ignored and disparaged here = their observed “… pause in warming”.

The implication of both papers is that climate models face challenges when compared to real world observations. Since accurate modelling of natural variability is essential if we are to move on to determine the extent of warming caused by humans and possible mitigation scenarios, I’d be more comfortable with the demands for massive mitigation efforts and the claims of impending catastrophe if we could at least have models that matched up to reality in a more robust fashion.

Your ongoing double standard for “citations” supports my point also – you and others routinely attack people here on general grounds, yet suggest the general criticism of others is misguided. Citation?
[edit – OT and wrong in any case]

Is it known if the PETM had any sort of cellular structure, analogous to but presumably different from the current day Polar – Ferrel – Hadley structure? Do any of the GCMs reveal collapse of the PFH structure under future scenarios, or are such structures absolutely stable? If they show propensity for instability, what are the key triggers for destabilisation? If there was a significantly different cellular structure, or dyanamic cellular pattern/ordering existing through the PETM, might that be accompanied by strong spatial-temporal dependence of climate sensitivities (and hence accompanying geo/bio phenomena)?

While appreciating Dr. Dickens comments on this thread, I do have a problem with his statement that has caused such a kerfuffle i.e.:

In a nutshell, there appears to be something fundamentally wrong with the way temperature and carbon are linked in climate models.

This seems ill-advised for two (linked) reasons:

1. We don’t really know what the situation was at the time of the PETM 55 MYA. Perhaps when the situation is clearer we’ll find that the way temperature and climate is linked in climate models actually applies very well to the PETM.

2. There’s no reason why climate models, parameterized with respect to the wealth of detail of Holocene and late Quaternary measurements and proxies, should necessarily apply to the Cenozoic era. The world was very different then. The apparent lack of expected response of Cenozoic era climate to greenhouse forcing then doesn’t necessarily mean that there is a significant problem with current modelling of climate responses to greenhouse forcing now .

Joe, Swanson (and Tsonis) didn’t “observe” “a pause in warming”! They postulated the possibility of a pause in warming….

…and the Zeebe paper doesn’t “impl(y) that climate models face challenges when compared to real world observations”. After all, when we fully understand the PETM, we might find that climate models fit the observations rather well after all. Alternatively, there’s no reason to expect that climate models parameterized with respect to Quaternary observations/proxies, should necessarily apply to the Cenozoic era which was a rather different world.

Your post highlights exactly the problem with the rather careless press release from Rice University. It is catnip to misrepresenters!

Of course we should continue to improve climate model parameterization by comparison with real world observation. But neither Swanson and Tsonis, nor Zeebe et al. are necessary grounds for questioning current climate models. If anything, Zeebe et al might lend us to consider that unforeseen positive feedbacks could make climate responses to greenhouse forcing rather more problematic than models project. That’s what Dr. Dickens was trying to convey in the the press release I believe.

A question for any of the researchers familiar with the paleo record —

> the most prominent victims were deep-ocean microscopic foraminifera
Is there any estimate of the biomass and CO2/methane they’d release, dying?

And, of the species that didn’t disappear, how about the biomass of those that lived mostly in the depth range of the expanded zone of solution — same question, are the biologists contributing to the search for possible added CO2 and CH4?

“…what would have happened to the carbon you’re talking about? Is there a spike in accumulation of some carbon-containing material (sedimentary rock, presumably?) associated with the end of the PETM?”
Hank Roberts — 13 August 2009 @ 1:25 PM

“Initial results suggest that the Eocene cooling occurred much earlier, and much faster than previously assumed; summer SSTs were extremely high during the so-called Paleocene Eocene Thermal Maximum (ca. 20-24ºC, at ~55 Ma) dropping to values as low as ~5ºC only 5 Ma later. These values are in good agreement with initial dinoflagellate-based SST-trends. One hypothesis is that this dramatic cooling that happened much faster than up to now anticipated may be related to a substantial drop in pCO2 resulting from the highly efficient production and burial of large amounts of organic carbon in the huge Arctic Basin. This may have been the consequence of anoxic water column conditions in the stratified basin and the massive growth of the hydropterid fern Azolla in the surface waters of the Arctic at this time, another surprising ACEX finding.” http://www3.bio.uu.nl/palaeo/research/MarineSystems/index.html

“ТACEX was the first Integrated Ocean Drilling Project (IODP 302) expedition into the Arctic and recovered over 400m (1,400ft) of cores, including 200m (700ft) of Paleocene and Eocene deposits.’ explains Jonathan. ‘In the section corresponding to the earliest Middle Eocene – near the point in time when we start to see a sudden shift in CO2 levels – we found more than eight metres of core composed almost completely of a plant known as Azolla, a floating fern sometime found in suburban ponds.”
GEO ExPro September 2007

“An Azolla Event in the Middle Eocene – Dinoflagellate cysts, diatoms, ebridians, and silicoflagellates are common to abundant in the middle Eocene section that includes a spectacular basal layer showing massive occurrences of glochidia and massulae (megaspores) of the fresh-water hydropterid fern Azolla, suggesting strongly reduced surface-water salinity or perhaps even a brief episode of fresh-water conditions at the surface.” http://www.iodp.org/iodp_journals/2_IODP_Expedition_302_SD1.pdf

I plugged some very approximate numbers into a spreadsheet and came up with about 900 gigatons of carbon sequestered, so the order of magnitude is about right(i.e., 90 Gigatons wouldn’t do it, and 9000 Gt would be too much); I wouldn’t bet on my numbers being within an order of magnitude, though. Well, maybe a sixpack of Guiness; I could afford to lose that.

Well, from the little ive read of this event, the most likely mechanism for the shift may be a submarine tectonic rift/volcanism. And until the ocean circulations can be modeled with sufficient resolution for that time period, i doubt whether the models will ever be able to recreate this event accurately.

Obviously its the continents, and their effect on ocean circulation/albedo that set the initial climate, and considering the relative stability of the various other “hot house” periods, it stands to reason that the trigger for this event, is something on the exceptional side. As opposed to a gradual tipping point being reached?

How well is ocean circulation modeled? It seems like this would be near impossible to do in hind site?

With all due respect, I think you are missing my main points (although given all the *kerfuffle* — great word btw — it’s certainly possible that I’m just not very good at making points clearly!). So, to clarify and to address several other comments, I give the following with a bit more elaboration (apologies if this is a long boring post, and I hope that people don’t pick and choose selected passages) …

1/ In regards to using Quaternary (which includes the Holocene btw) climate variations as a template for constraining climate models for the future, there are two basic and somewhat related problems. One of these is observational; the other is conceptual. There are no clear examples in the pre-industrial Quaternary where massive amounts of carbon entered the exogenic carbon cycle (i.e., the combined ocean-atmosphere-biosphere). Yes, of course, there were definitely major changes in climate (and surface temperature) across Earth, and, yes, of course, some of these are obviously linked to changes in atmospheric pCO2. But nowhere in this part of the geological record is there compelling evidence for a warm climate (i.e., similar to today) when large quantities of carbon rapidly entered the exogenic carbon cycle from an external source (e.g., fossil fuels). Rather, in regards to Quaternary climate variations, the general consensus is that carbon was redistributed between the ocean, atmosphere and biosphere because of external forcing (e.g., changes in Earth orbital configurations). However, to my knowledge, present climate models do not have carbon fluxes as internal components to Earth’s system that respond to external forcing (i.e., the carbon fluxes are prescribed during time runs – but please correct me if I am wrong here because I have to edit these sort of papers pretty regularly). (Also, as an aside, I should stress that there are definitely ideas for where and how the carbon is redistributed). Now, one might suggest, as some of my friends and colleagues have done, that these things do not matter in terms of predicting future climate change for the next 100 years, and this may, in fact, be the case. But, please understand, there are some basic issues with trying to use observed climate change during the Quaternary to constrain existing models for predicting future climate change (i.e., adding carbon to a world where no such analog exists in last million years (probably 40 million years) and the models are not appropriately set for this anyhow (e.g., I agree with Bryan’s comments btw).

2/ The amazing aspect of the PETM is that this extreme warming event is somehow associated with a fairly rapid and massive input of carbon to the exogenic carbon cycle from an external source. There is actually a great deal of information about the PETM, although there are also some major problems, again both observational and conceptual. There are the basic facts: we (well we think) we know the approximate temperature rise (~6 °C based on d18O, Mg/Ca, TEX86 and fossils – I can elaborate on these proxies if needed …); we also know there was a massive input of isotopically-depleted carbon (i.e., carbon depleted in 13C, similar to fossil fuels). This latter fact is readily appreciated by looking at hundreds of locations across the globe, which show a carbon isotope excursion and, in the deep-sea, carbonate dissolution. The event is wholly unlike anything in the Quaternary (and, in fact, probably at least the last 90 million years in terms of magnitude, except, of course, what is happening now). The overarching problems are that we do not know where the carbon came from (i.e., its source, mass and composition) or how it entered the exogenic carbon cycle, although there is much speculation (e.g., oxidation of peat, release of methane from seafloor, contact metamorphism of a petroleum system, etc.).

The placement of *associated* (above) is deliberate. The latest IPCC document *Scientific Basis for Climate Change* also nicely (and fairly) dances around this issue when discussing the PETM (p. 442). The gist behind the semantics is that some data suggests that other forcing may have initiated the carbon input (i.e., it was not truly an external input of carbon, such as from the burning of fossil fuels, a carbonaceous bolide impact, contact metamorphism, etc.); rather, it involved a positive feedback. This complication, however, is subordinate to the case that the PETM was almost assuredly linked to an external source of carbon to the ocean/carbon/biosphere system, unlike anytime in the Quaternary. (To elaborate on two possibilities in this regard – initial changes to Earth’s coupled system drove warming of bottom waters and released seafloor methane or drove a change in the hydrological cycle and oxidation of peat).

To restate earlier posts (and my quotation that led to all of this), no one has been able to explain basic temperature observations before, during, and after the PETM using a climate model. Indeed, with most existing carbon cycle models, there is no way to actually explain the event … despite the fact that it happened. (But to stress again, observations of the rock record during this time point to a greater temperature rise per unit of carbon added to the system than suggested in most existing models).

(As an aside to Bryan, the beauty of examining the PETM from a scientific perspective is that the input (and output of carbon) occurred very fast (geologoically). There is likely no need to consider long-term changes in boundary conditions, although the precise boundary conditions at the time are important. My main point here, we probably do not need to model the entire Cenozoic to understand a short-term perturbation to the system)

3/ From perhaps an overly simplistic view, one might consider the present input of carbon as one where the Quaternary is appropriate for setting boundary conditions (e.g., continent positions, ocean circulation, overall distributions of carbon, permutations to orbital configurations, etc.) but that carbon injection events of the early Paleogene (e.g., the PETM) are appropriate for understanding the effects of external carbon forcing (even if the carbon is a coupled external feedback). Both have conceptual issues, both are important. There is certainly a tendency for people to dismiss something that happened 55 million years ago as too distant to be used for model constraints, especially when the observations do not conform to expectations. But this is neither helpful nor good science. We have the basic facts that (a) there was a brief interval in time (the PETM) where global temperatures rose significantly and almost coincident with a massive input of carbon to the ocean and atmosphere; but (b) we cannot understand the link between temperature and carbon input during this time with existing models, either in terms of causal relationship or magnitude. Something needs to be amended (and, btw, just changing 1 to n parameters for climate sensitivity in models, while perhaps acceptable in terms of computer coding, does not give a lot of insight to the basic problems).

4/ So where does this leave us as a scientific community studying climate change? Without doubt, from short to long records of the geological record (at least over the last 100 million years), there is almost unquestionably a link between Earth surface temperature and atmospheric pCO2. Do we have climate (and carbon cycling) models that can satisfactorily explain these links? I would argue no (and I will suggest that many of my colleagues would agree on this but I’ll let them dare enter into this crazy hornet’s nest of blogosphere politics, science and media ….). Does this mean that climate models predicting climate change on the 100 yr time scale (the immediate future) are wrong? I don’t know, because this may be an issue of time and a straightforward issue for atmospheric scientists, and where my expertise fades. I would strongly suggest, though, that the geological record indicates that there are carbon cycle feedbacks that are not incorporated in most models for climate change, at least as we can observe on the >1000 year timescale. Okay, I can see how that comment can spun all sort of ways … I’m learning ☺

I hope this addresses things more clearly, although maybe I am just dreadful at the role of presenting science to people ….

In response to Sidd (165) and others with some great science questions – (these are definitely more fun to consider than comments revolving around semantics, notions, emotions, politics, etc).

Well, probably the simplest and most straightforward means to answer the science questions that have arisen in this stream is to pass along a review paper from last year (Zachos, Dickens & Zeebe, Nature, v. 451, p. 279-283). It can be downloaded at:http://www.nature.com/nature/supplements/collections/yearofplanetearth/
(Although, I am not sure if one needs a subscription to access).

They mention that the warm poles could not be due entirely to increased poleward heat flux in ocean currents, and speculate that the heating influence of polar stratospheric clouds may have played a role. Can anyone think of some paleo measurement that might confirm or refute an increase in such clouds during the PETM ? I confess that I cannot, but I hope I am wrong.

[Response: No. The problem with the polar stratospheric clouds idea is that it has become clear that getting the requisite cloud optical thickness to make a difference is currently impossible using any known or easily extrapolatable physics. So while you can hypothesise a LW absorber in the stratosphere that does the trick, no one has any mechanism to actually produce one. Nonetheless, something was going on – and people are trying lots of different ideas to produce a similar result (no luck so far though). – gavin]

Thanks to Dr. Dickens and RC for this intriguing post and follow ups, which are clearly written and well articulated. I think the two very different types of responses to the paper and comments are really intriguing and suggest many are a lot more interested in position advocacy than more rational approaches.

Thanx for the response. I see from the Huber reference in the Zachos paper that an oceanic heat flux on the order of 1 Petawatt is insufficient to reduce the equator to pole temperature gradient. I calculate that Petawatt scale fluxes due to precipitation into the polar regions would require rainfall on the order of 10meter/yr. I take it that there is no evidence for such huge precipitation imbalance in the record, at least I have not been able to find much.

Many thanks for your comprehensive account Dr Dickens. If the press had access to that level of explanation/interpretation there would certainly be no possibility of misinterpretation! I don’t disagree with any of what you say. In fact the main point I raised is incorporated in your statement about your colleagues and their assessment that current climate models may be rather good for assessing the climate response to raised greenhouse gases over the coming century. I would tend to go along with that. The concern that is reinforced from your studies of the PETM is the very real (but presently unparameterizable perhaps) possibility of downstream feedbacks that might greatly amplify the warming. I guess the most well understood of these might be the warming-induced release of methane from tundra and clathrates and other currently sequestered carbon sources (e.g. peat or forest).

Can I ask a question about your paper that concerns methane feedbacks? In both your paper and the accompanying commentary of Beerling, methane release from unknown sources is proposed as one of the putative feedback that might have amplified the warming resulting from the initial 3000 Pg release (and the subsequent longer term “pulsed” release of carbon) that initiated the PETM. Since this methane (from clathrates or swampland or permafrost) will also be 13C-depelted and presumably (once oxidised) will contribute to the effects on the carbonate-dissolution records you used to constrain the amount of carbon released, shouldn’t this “feedback” (warming-induced methane release) have already been accounted for in your analysis, if it had in fact existed?

Or does your analysis only constrain the initial (~3000 Pg) carbon release, and any subsequent release (e.g. 13C-depleted C from methane) is not directly assessed in your analysis? I’ve read your very nice paper a couple of times, but can’t clarify this point myself!

“Now for the Paleocene, it is unlikely that changes in ice sheets were very relevant (there weren’t any to speak of”

This view (Shackelton and Kennett, 1975) is coming under more and more scientific scrutiny in recent years. In 1980, Matthews in Poore published a paper suggesting that glacio-eustacy was the dominant driver in sharp rapid sea level changes for much of the Paleogene, even back into the Cretaceous. This hypothesis resulted from examining the relationship between the d18O proxy and the global sea level curves produced by Exxon in the late 1970’s (credited largely to Peter Vail, 1977). The hypothesis went largely un-noticed outside the petroleum geology community, and the dominant theory held that Antarctic glaciation was not initiated until the middle Miocene (ca 15 Ma). With the publication of the Haq et al. sea level curves in 1987, it came under wider recognition that large and rapid eustatic changes, if valid, presented a serious challenge to the theory that the Eocene and Paleocene were ice free. It is important to note that such rapid and large magnitude changes reported in the Haq et al. curves can only be explained by glacio-eustacy. The Haq paper was highly criticized within academia, mainly due to the lack of public access to much of the proprietary seismic data used to generate these curves, and also the widespread view that these curves were contaminated from local effects. The Matthews hypothesis continued to go largely un-noticed, except by a few workers (Stoll and Schrag, 1998) (Abreu and Anderson, 1998) (Miller et. al 2005). In recent years however, the Haq curves have been independently produced by several workers, and largely vindicated. An important recent paper by Miller et al. in 2008 looked at the passive margin along the coast of New Jersey, and demonstrates that the first order sea level cycles agree well with the Haq curves, although the absolute magnitude of the variations is somewhat less than earlier reported (although still very significant). The new work also demonstrated a good correlation between sea level and the d18O proxy, as had been asserted by Abeu and Anderson in their 1998 paper.

This newer emerging view of glacio-eustacy is of importance to the problem of investigating even rapid events such as the PETM. First, temperature reconstructions for the Early Cenozoic are based largely on the d18O proxy, and implicit in the older reconstructions was the assumption that ice sheets during the early Cenozoic are negligible. Since the d18O proxy is sensitive to both temperature and glaciation, the temperature story gets very complicated in the presence of ice. Secondly, significant ice sheets in the hothouse climate of the Eocene, even if ephemeral, are potentially important to the feedbacks within the earth system. As Gavin states in his essay, modelers currently view these ice sheets as either non-existent, or too small to matter. That older assumption is not holding up well in recent years however. The initiation of both Antarctic and even NH glaciation is being pushed further and further back in time. An examination of the Miller (2008) curves shows rapid sea level rises and falls of over 50-75 meters on either side of the PETM. These curves present a significant challenge to anyone who insists on largely ice-free conditions near the Paleocene-Eocene boundary.

[Response: There’s a big difference between the early Cretaceous and the Paleocene/Eocene transition period, let alone the mid Eocene thermal max. I am unaware of any evidence for significant ice over that period (say 53 to 45 million years ago) – but maybe I just haven’t noticed. Do you have anything specific? – gavin]

Gavin, Yes, there is specific evidence for ice at the Paleocene/Eocene boundary and also near the Eocene Climatic Optimum (ca 51 Ma). The new sea level curves shown by Ken Miller indicate 50 meter+ rises in falls of sea level within relatively short intervals surrounding the Paleocene/Eocene boundary, and also periodically during the ECO. Such rapid sea-level changes pose an enigma, because the only known mechanism for causing sea-level changes in excess of 10m in less than 1myr is glacioeustasy (Golovchenko, 1983).

Specific sedimentological evidence for glaciation in East Antarctica during the Eocene comes from ODP Leg 119 drill sites on the continental shelf of Przdz Bay. Site 742 in Prydz Bay recovered middle-upper Eocene massive diamictons, interpreted as water-lain till (Barron et al., 1991, Hambrey et al., 1991). The occurrence of till on the continental shelf is synchronous with the first significant occurrence of ice-rafted detritus at Leg 119 site 738 on the Kerguelen Plateau (Ehrmann, 1991, Abreu and Anderson, 1998). Evidence also exists for alpine glaciation in West Antarctica during the middle Eocene. Glacial marine sediments were recovered at CIROS-1 in the western Ross Sea which contain middle Eocene dinoflagellates (Hanna, 1994). The evidence is however, that the West Antarctic glaciation did not advance across the continental shelf during the Eocene.

Gavin, the evidence is quite compelling that there is significant ice on the continent possibly coinciding with some of the warmest hothouse climates of the Eocene. I encourage you to read some of the references I have listed here. John Anderson, who is a respected researcher of Cenozoic Antarctic geology (and a colleague of Gerald Dickens at Rice) has compiled some of this data in his paper with the reference listed above.

“… The sediment records are studied to gain insights into palaeo glacial dynamics of the East Antarctic Ice Sheet from a marine perspective by the reconstruction of glaciomarine processes in the past. The study area is bordered by the Lambert Glacier-Amery Ice Shelf system that drains about 20% of the East Antarctic Ice Sheet and is considered to be representative for the behaviour of the whole East Antarctic Ice Sheet. The retrieved sediment cores allow the reconstruction of ice-sheet dynamics at different time scales, comprising high-resolution Holocene records, the time interval of the latest glacial-interglacial cycle during the past 130 kyr, and one long-term record back to the mid-Pliocene. Special emphasis will be focussed on the provenance and dispersal of ice-rafted debris (IRD) and the distibution of contourites in space and time, as indicators of palaeo iceberg drift tracks and AABW activity….”

What difference does the rate of change (of rate of change) make, if any, with feedbacks other than ocean pH? I think I understand that with a geologically typical CO2 increase, the ocean pH doesn’t change very much because other chemical reactions use up the ions as fast as the CO2 dissolves. But with our extreme rapid increase of CO2 that doesn’t work, so the carbonic acid accumulates.

I know there were some suggestions a comet or asteroid impact into an area rich in carbon could kick off something like the PETM event if it happened on top of a normal warm stretch. Anything more on that?

I’ve added a few more references from my comment #185 and fixed the format for the previous ones listed. Many papers and reports (many over 20 years old) documenting evidence of at least ephemeral Antarctic ice sheets during the Eocene have not been widely read outside the geology community, and it is my hope that this accumulating body of information in support of the Matthews hypothesis will become better appreciated in the larger earth systems world. For climate scientists and modelers without a strong working knowledge of seismic stratigraphy, a good place to begin is with the 2008 Miller et al. paper and work backward to some of the older literature.

1) Is it possible that albedo feedback might have been a larger contributor ?

2) Would a larger albedo feedback help explain either the magnitude of temperature rise or low pole to equator temperature gradient ?

3)A letter to Nature in 2006 by Nunes and Norris indicates that ocean overturning and deepwater formation switched from S to N hemisphere at the beginning of PETM. Has there been further work in that area ?
(Nature Letters v 439, pp60-64, 2006)

Abrupt increase in seasonal extreme precipitation at the Paleocene-Eocene

—-

Now again the PETM was fast the way geology is fast, not fast the way the anthropocene event is fast.

But does this suggest we might not see ocean pH change as projected, because we might see a rapid increase in “weathering” as in peak storm erosion increasing available nutrients in the ocean? Not to mention all that fertilizer ….

Following citing papers — oh my, the further in you go the bigger it gets:

http://www.pnas.org/content/105/46/17595.full
In this issue of PNAS, Schumann et al. (4) report evidence for new microorganisms that appeared and disappeared with the PETM, signaling another specific ecological response to the biogeochemical changes associated with this extreme warming event….

Nature doesn’t throw adaptations away, so odds are the genes highly favored during the PETM are conserved at low levels.

Geez Climate scientists – and science journal editors in particular – have to realise the political and social context of their work. If you’re studying an obscure subspecies of termite then choose your own title. But if the future of civilization depends on responding urgently to Global Warming – to give momentum to global warming deniers at this point of time by loose phraseology in paper titles is foolish, frustrating and unethical.